The little guys often get pushed around by the big guys. In the case of a planet, the "pushing" can have a big effect on the quality of life. In fact, it can determine whether a planet can sustain life at all.

For a planet to sustain life as we know it, it must reside in a region known as the habitable zone. It's the distance from the parent star where temperatures are just right for liquid water.

In young solar systems, it's hard for an Earth-like planet to find that zone -- especially if it has larger siblings that follow elongated orbits. As a bigger planet orbits the star, its gravity pushes and pulls on the Earth-like planet -- perhaps shoving the planet into an elongated orbit of its own.

As a result, the planet may spend part of its time inside the habitable zone, and part of its time outside. If the planet snuggles too close to its star, it gets too hot -- the water on its surface boils off. And if the planet moves too far from the star, it gets too cold, so water freezes. Either way, it's difficult or even impossible for life to arise.

And that may be what's going on in many of the planetary systems that astronomers have discovered. Many of these systems have giant planets in stretched-out orbits. No Earth-like planets have yet been found in these systems. But if such worlds exist, the pushing around they get from the giant planets may make it tough for them to support life.